Lee Fulton, #129837, is the modera-
tor on the Flying Bricks K bike forum. Lee owns six K bikes and is a retired maintenance machinist who enjoys riding and wrenching. Possessing extensive knowledge of early K bikes, Lee shared advice that helped me suc- cessfully complete my project. After sharing the mouse nest dis-
covery on the forum, Lee responded, “The air filter should have kept any debris caused by the mouse from affecting the air flow meter itself, but if enough debris was present under the filter, it could affect running. I would expect it to affect high rpm first just by virtue of reduced airflow capacity. The fact that the motor runs poorly at low rpm, but fine at high rpm suggests good fuel and airflow and good spark (If your spark plug gap is correct. Too large a gap will have trouble firing at low rpms). Another possibility is exhaust valves that are too tight. If the plugs are good and gapped correctly, the exhaust valves are correctly adjusted, and the fuel pressure is correct, it suggests a
vacuum leak, or more accurately, unme- tered (unseen by the computer) air entering the intake tract. If more air is entering the cylinders than the computer knows about, the computer does not inject the appropri- ate fuel and the bike runs very lean. Since the excess air remains relatively constant throughout the rpm range, the proportion of excess air at low rpm is high and the pro- portion of excess air at high rpm is low. The bike will run poorly at low rpm and okay at high rpms. The possible sources of this vacuum leak
are (roughly in order of probability or ease/ cost of correcting): 1. Crankcase breather hose and be sure to use an OEM hose from BMW (originals can go bad in 1-2 years). They will crack just below the upper and just above the lower hose clamps.
2. Throttle body vacuum caps on #1 and #2 cylinders (+#3 on K 100s).
3. An unplugged or cracked hose or plug to the nipple at the front top of the engine just below the fan. This is where the fuel tank was originally vented. In re-venting, many previous owners either did this wrong or poorly so that a leak develops.
4. The oil filler O-ring. This O-ring never gets any thought, but it hardens and then instead of O-ring to crank cover you get plastic cap to cover with some air leak- age. Cheap insurance - if you feel the cap hit the crank cover when you tighten your oil filler cap, replace the O-ring.
Now we get to the things that take more
time and money to replace. 1. The vacuum line between the #3 (or #4 on a K 100) cylinder and the fuel pressure regulator. This is just bulk vacuum hose, nothing special.
2. The 2-1/2” rubber elbow between the air flow meter and the intake plenum. If this hose has any cracks, it will cause a major running problem.
3. The throttle body upper and lower boots (manifolds). If any one of the six (or eight on a K 100) is bad, replace them all because of the labor involved to get at them.
4. A bad fuel pressure regulator. This can either cause a vacuum leak because of a bad diaphragm, or it can cause too high a fuel pressure which COULD cause the exact problems that you are experiencing
Left, Dirty throttle bodies before work began. Above, New and existing parts await instalation.
48
BMW OWNERS NEWS February 2016
TECH
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